The electrolytic and electroless plating of silver on various substrates has a long history. One of the applications of electroless plating is in the area of oxygen electrodes, e.g., in fuel cells and electrolytic cells. In recent times, U.S. Pat. No. 3,864,236, granted Feb. 4, 1975, disclosed an air cathode having two porous layers, the coarse or inside layer containing commingled catalyst which may be silver. The air cathode is said to be made of a mixture of catalyst, carbonyl-nickel powder and spacer which are pressed and sintered together to a desired porosity. The fine layer, exposed to the catholyte, does not contain catalyst. In preparation, the fine layer which does not contain catalyst or spacer is first put into a mold. Then the coarse layer powder is put on top of the fine layer followed by sand and the composite is pressed together, e.g., at one ton/cm.sup.2 (1000 kg/cm.sup.2).
F. Pearlstein and R. F. Weightman, Plating 61, 154 (1974) [following up their earlier report in Plating 58, 1014 (1971) and that of N. Feldstein, RCA Review 31, 317 (1970) on the catalyzed reduction of silver ions by dimethylamine borane] reported on an electroless silver plating technique using 2.0 g/l dimethylamine borane, 1.34 g/l AgCN, 1.49 g/l NaCN (wherein NaAg(CN).sub.2 is formed in excess of NaCN) and 0.75 g/l NaOH in distilled water. Deposition at a constant rate was described for 10 cm.sup.2 solid silver panels which were soak alkaline cleaned, rinsed, immersed in aqueous sodium hydroxide and sodium cyanide mixture to remove tarnish, rinsed, immersed in aqueous dilute palladium chloride-hydrochloric acid solution to insure presence of an active surface for initiating electroless deposition, rinsed, dried and weighed. Silver was thereafter deposited on the prepared silver panels using 500 ml of the solution described above for one hour at 55.degree. C. at a rate of about 2.5 .mu.m/hr.
Bregoli et al., in Canadian Pat. No. 921,111, granted Feb. 13, 1973, disclosed a controlled potential pulse plating method. In this electroplating method, a potential is applied to a porous electrode structure (the cathode) which is immersed in a plating solution which can be a solution of a silver compound. The potential is applied in pulses by a complicated apparatus which would appear to be difficult to apply routinely. The porous electrode structure is catalyst activated after fabrication, which may be by sintering or otherwise, to give an electrode suitable for fuel cells.
Schulmeister et al., in U.S. Pat. No. 3,787,244, granted Jan. 22, 1974, disclosed a porous or sintered fuel cell electrode which is catalyzed by a replacement plating process. The substrate may be porous nickel. Noble metal ions from a dissolved salt replace a thin layer of the nickel surface within the pores. In their process, the solution containing one or more noble metal salts is poured on top of the porous electrode and a partial vacuum is drawn on the bottom side of the electrode.
Schroeder et al., in U.S. Pat. No. 3,539,469, granted Nov. 10, 1970, disclosed a fuel cell cathode catalyst comprising a nickel powder, the particles of which are coated with silver. Nickel powder is immersed in dilute acid to activate its surface and a silver salt is then added to the acid solution whereby metallic silver replaces surface nickel. The silver-coated nickel powder is then washed and heat treated, then compacted into an electrode.
Zirngiebel and Klein, in U.S. Pat. No. 3,140,188, granted July 7, 1964, disclosed that certain borazane or borane compounds having the given structure R.sub.3 N-BH.sub.3, wherein R is said to denote an alkyl or aryl radical or H, then denominated borazanes and including, for example, N,N,N-trimethylborazane, N,N-dimethylborazane, N-methylborazane, N,N-diethylborazane, N,N,N-triethylborazane, are useful as reducing agents in electroless baths for plating iron, cobalt, nickel and zinc on appropriate plastic films or metal sheets.
Berzins, in U.S. Pat. No. 3,338,726, granted Aug. 29, 1967, also found such amine boranes, particularly the tertiary amine boranes and including pyridine borane, useful as reducing agents in electroless plating solutions containing nickel and cobalt ions for producing nickel-boron and cobalt-boron alloy plates.
Mallory et al., in U.S. Pat. No. 3,597,267, granted Aug. 3, 1971, also found that the amine boranes such as dimethylamine borane and morpholine borane are useful as reducing agents in a bath for the electroless plating of Ni, Co, Fe and Cr on Bi, Cd, Sn, Pb and Zn substrates.
It is desired to provide a ready method for depositing accurately known amounts of silver over substantially the whole exposed surface of a permeable substrate.